Mydriasis revealing vascular and osteodural compression of the oculomotor nerve: An observational study on five cases.

PURPOSE: To describe a form of neurovascular compression of the third cranial nerve (CNIII) in idiopathic mydriasis, in which a neurovascular "conflict" exists between the oculomotor nerve, the posterior communicating artery and the clinoid process, using high-resolution magnetic resonance imaging (MRI) with fast imaging employing steady acquisition (FIESTA) sequences.

METHODS: An 18-month prospective, observational and monocentric case series report was performed. MRI was performed on 5 consecutive patients with idiopathic, unilateral, persistent and nonreactive mydriasis (pure intrinsic palsy of the CNIII). Patients with diplopia, ptosis or ophthalmoplegia were excluded. Cerebral MRI focused on the CNIII pathway from the mesencephalon to the cavernous sinus entry, particularly on the cisternal segment: image acquisition was performed on a 3 Tesla MRI; the protocol included fast imaging employing steady acquisition (FIESTA) and three-dimension time of flight (3D TOF) sequences.

RESULTS: All patients presented a neurovascular compression point, involving the CNIII clamped between a tortuous posterior communicating artery (PCoA) and the posterior clinoid process at the entrance of the cavernous sinus. No cases occurred in the root entry zone. There was no compression on the contralateral side. No tumors or aneurysms were found. Thus, the mydriasis was caused by CNIII compression.

CONCLUSION: MRI, including FIESTA sequences, revealed a new type of neurovascular conflict between the CNIII, PCoA and posterior clinoid process in patients with incomplete oculomotor palsy. Non-aneurysmal CNIII compression should be considered as a differential diagnosis in the work-up of idiopathic mydriasis. The role of MRI in the work-up of anisocoria should be considered.